Method of re-fracturing using borated galactomannan gum

Abstract

A well treatment fluid containing borated galactomannan may be used to isolate a productive zone in a well having multiple productive zones. The fluid is particularly useful in treatment of wells containing a mechanical zonal isolation system in the productive zone of interest. The fluid is pumped into the well in a substantially non-hydrated form. The well treatment fluid is therefore highly effective in preferentially sealing or blocking productive zones in the formation since delayed hydration of the fluid may be controlled for up to several hours. The seal may be degraded and a productive zone subjected to re-fracturing by introducing a viscosity reducing agent into the well.

What is claimed is: 
     
       1. A method of re-fracturing a subterranean formation penetrated by a well having multiple zones, the method comprising:
 (a) subjecting a productive zone within the subterranean formation to hydraulic fracturing; 
 (b) isolating the fractured productive zone from a second zone in the well by (i) pumping into the well a treatment fluid comprising an unhydrated borated galactomannan gum and a crosslinking agent, wherein the unhydrated borated galactomannan gum prior to being crosslinked contains borate ions; (ii) forming a gelled temporary seal by reacting the unhydrated borated galactomannan gum with the crosslinking agent and thereby isolating the fractured productive zone from the second zone; 
 (c) degrading the gelled temporary seal between the isolated fractured productive zone and the second zone by pumping into the well a viscosity reducing agent and reducing the viscosity of the gelled temporary seal with the viscosity reducing agent, wherein the viscosity reducing agent is pumped into the well at a pressure insufficient to create or enlarge a fracture within the subterranean formation; and 
 (d) re-fracturing the isolated fractured productive zone after degradation of the gelled temporary seal by pumping into the well a fracturing fluid at a pressure sufficient to create or enlarge a fracture within the isolated fractured productive zone. 
 
     
     
       2. The method of  claim 1 , further comprising:
 (e) pumping into the well near the re-fractured isolated fractured productive zone of the well a well treatment fluid comprising an unhydrated borated galactomannan gum and a crosslinking agent, wherein the unhydrated borated galactomannan gum prior to being crosslinked contains borate ions; and 
 (f) isolating the re-fractured isolated fractured productive zone from another zone of the well by hardening the well treatment fluid of step (e). 
 
     
     
       3. The method of  claim 2 , wherein steps (a) through (f) are repeated at least once. 
     
     
       4. The method of  claim 1 , wherein the well is a vertical well. 
     
     
       5. The method of  claim 1 , wherein the viscosity reducing agent is an acid. 
     
     
       6. The method of  claim 5 , wherein the acid is selected from the group consisting of hydrochloric acid, formic acid, sulfamic acid or a mixture thereof. 
     
     
       7. The method of  claim 1 , wherein the viscosity reducing agent is an oxidative breaker or an enzyme breaker or a combination thereof. 
     
     
       8. The method of  claim 7 , wherein the viscosity reducing agent is selected from the group consisting of alkaline earth metal peroxides, metal peroxides, organic peroxides, hydrochlorite bleaches, persulfate salts, chromous salts, sodium bromate, sodium perchlorate, sodium perborate, magnesium perborate, calcium perborate and galactomannans and mixtures thereof. 
     
     
       9. The method of  claim 1 , wherein the subterranean formation is shale. 
     
     
       10. The method of  claim 1 , wherein the well is a horizontal well. 
     
     
       11. A method of fracturing a subterranean formation penetrated by a well comprising:
 (a) pumping into the well an unhydrated borated galactomannan gum and a crosslinking agent; 
 (b) sealing a fractured productive zone from another zone in the well with a temporary blocking gel by reacting the borated galactomannan gum and crosslinking agent to form the temporary blocking gel from the reaction product; 
 (c) pumping into the well a viscosity reducing agent, reducing the viscosity of the temporary blocking gel, unsealing the fractured productive zone from the another zone and removing the temporary blocking gel from the well, wherein the viscosity reducing agent is pumped into the well at a pressure not sufficient to create or enlarge a fracture within the subterranean formation; and 
 (d) re-fracturing the unsealed fractured productive zone by subjecting the unsealed fractured productive zone to hydraulic fracturing by introducing into the unsealed fractured productive zone a fracturing fluid at a pressure sufficient to create or enlarge a fracture; and 
 (e) pumping unhydrated borated galactomannan gum and crosslinking agent into the well and forming a temporary blocking gel between the re-fractured zone and the another zone by reacting the unhydrated borated galactomannan gum and crosslinking agent. 
 
     
     
       12. The method of  claim 11 , further comprising repeating steps (a) through (e) in one or more productive zones of the well. 
     
     
       13. The method of  claim 11 , wherein the well is a vertical well. 
     
     
       14. The method of  claim 11 , wherein the viscosity reducing agent is an acid. 
     
     
       15. The method of  claim 11 , wherein the viscosity reducing agent is an oxidative breaker or an enzyme breaker or a combination thereof. 
     
     
       16. A method of enhancing the productivity of a hydrocarbon-bearing formation penetrated by a well having multiple productive zones comprising:
 (a) forming a temporary blocking gel between a previously fractured productive zone and another zone by pumping into the well an unhydrated borated galactomannan gum and a delayed crosslinking agent and reacting the unhydrated borated galactomannan gum and delayed crosslinking agent; 
 (b) pumping a viscosity reducing agent into the well, reducing the viscosity of the temporary blocking gel and then removing the temporary blocking agent from the well; 
 (c) pumping into the well a fracturing fluid at a pressure sufficient to initiate or enlarge a fracture within the subterranean formation and re-fracturing the previously fractured productive zone; and 
 (d) pumping into the well a well treatment fluid comprising unhydrated borated galactomannan gum and delayed crosslinking agent; and 
 (e) isolating the re-fractured productive zone by hardening the well treatment fluid comprising the unhydrated borated galactomannan gum and delayed crosslinking agent. 
 
     
     
       17. The method of  claim 16 , wherein the well is a vertical well. 
     
     
       18. The method of  claim 16 , wherein the viscosity reducing agent is an acid or a breaker or a combination thereof. 
     
     
       19. The method of  claim 16 , wherein the subterranean formation is a shale formation. 
     
     
       20. The method of  claim 16 , wherein the well is a horizontal well.